In recent years, fine processing is required for manufacturing semiconductor devices with higher degree of integration, and to attain the higher degree of integration, a shorter wavelength light source comes to be used in a light exposure apparatus used in photolithography. Up to now, the techniques of using KrF excimer laser radiation (248.4 nm) have been established, and a wide variety of resist materials suitable for this wavelength have been found. In Japanese Laid-open Patent Publication Nos. H9-6002, H9-6003, H5-249682, H4-219757, H2-209977, etc., various resist materials suitable for exposure with KrF excimer laser radiation have been reported. These known resist materials include polymers having at least a recurring unit represented by e.g. the following general formula (IV) and polyhydroxystyrenes or phenol resins having tert-butoxycarbonyl groups. ##STR1##
wherein R.sub.11 represents a hydrogen atom or a methyl group, R.sub.12 and R.sub.13 independently represent a hydrogen atom or a straight-chain, branched or cyclic alkyl group containing 1 to 6 carbon atoms whereupon R.sub.12 and R.sub.13 are simultaneously not hydrogen atoms, or R.sub.12 and R.sub.13 may be combined to form a methylene chain containing 2 to 5 carbon atoms, R.sub.14 represents a straight-chain, branched or cyclic alkyl group containing 1 to 10 carbon atoms, a straight-chain, branched or cyclic haloalkyl group containing 1 to 6 carbon atoms or an aralkyl group, R.sub.15 represents a hydrogen atom or a cyano group, R.sub.16 represents a hydrogen atom or a methyl group, R.sub.17 represents a hydrogen atom, a cyano group or --COOY whereupon Y represents a straight-chain, branched or cyclic alkyl group containing 1 to 6 carbon atoms, or R.sub.15 and R.sub.17 may be bound to each other to form --CO--O--CO--, p and q independently represent a natural number provided that 0.1.ltoreq.p/(p+q).ltoreq.0.9, and r represents 0 or a natural number provided that when r is a natural number, 0.05.ltoreq.r/(p+q+r).ltoreq.0.50.
Such resist materials, together with a photoacid generator for generating an acid upon irradiation to actinic radiation, and, if necessary, an organic base for stabilizing the line width of a pattern and additives for improving characteristics such as optical or mechanical properties, a film forming property, adhesion to a substrate etc., are dissolved in an organic solvent suitable for forming a resist coating to form so-called "chemically amplified resist composition". After the composition is applied to a substrate such as wafer etc., a resist image is formed.
The resist materials, for example represented by the above general formula etc. are synthesized by (1) reaction of (a) a homopolymer of hydroxystyrenes, a copolymer thereof with other monomers, or phenol resin and (b) a vinyl ether compound or di-tert-butyl dicarbonate, (2) homopolymerization of a reaction product from hydroxystyrenes and a vinyl ether compound or di-tert-butyl dicarbonate, or copolymerization thereof with other monomers, or (3) elimination, with an acid if necessary, of a part of functional groups introduced to these homopolymers or copolymers. To conduct these reactions, materials absorbing exposure radiation (e.g. aromatics)are often used as a reaction initiator acid catalyst. However the removal of this catalyst usually requires the step of isolating and purifying the polymer or the step of removing the catalyst, and after these steps, the reaction product is used as the resist material. The reason for the necessity of removing the acid catalyst by a certain method after the reaction is that this reaction is an equilibrium reaction so the remaining acid causes the reverse reaction to significantly deteriorate the performance and storage stability of the resulting photoresist. Further, if the polymer without isolated and purified is used directly to prepare a resist composition, the acid catalyst absorbs exposure radiation to cause unfavorable phenomena such as remaining of resist film after development. Generally to isolate and purify the polymer obtained by any of the preparation processes (1), (2) and (3) described above, the organic solution containing the polymer after the reaction is introduced into water to crystallize the polymer with stirring, and the precipitated crystals are recovered by filtration and washed with water under the present circumstances. And, if necessary, this polymer is dissolved again in a suitable organic solvent, the polymer solution is introduced into water, and the same procedure as above is repeated, followed by drying the precipitated crystals under reduced pressure, whereby the polymer suitable as a resist material (photosensitive material) is obtained. However, it is practically difficult to remove the acid catalyst from the polymer completely, thus permitting a part of the acid catalyst to remain in the polymer. And it causes a reduction in the degree of protection.
In the step of isolating and purifying the polymer as described above, about 5 L of organic solvent and about 150 L of water for crystallizing the polymer with stirring, and about 75 L of water for washing are the minimum requirement for producing about 1 kg of the resist material. Therefore, because of the problem of disposal of waste organic solvent and waste water, practical large-scale production as large as 100 kg batch or more is difficult, and it is also a great problem from the point of view of protection of the environment. Further, it was difficult to control the moisture content in each material in the reaction system up to now, thus leading to variations among lots in the degree of protection of the polymer. So it was necessary to measure the degree of protection after isolating and purifying the polymer, or to measure the moisture content in the reaction system to estimate the amount of a vinyl ether compound introduced into the reaction system in order to achieve desired degrees of protection.
Further, a polymer having a phenolic hydroxyl or carboxyl group protected partially is inherently chemically unstable due to its structure as compared with the usual polymer and undergoes decomposition reaction in the presence of a trace amount of acid or by heating. Decomposition products formed by this decomposition have great adverse influence on the performance of the resulting photoresist. In the conventional isolation and purification process, the elimination or decomposition reaction of the protective groups occurs depending on handling, and owing to variations in the degree of protection of the polymer, poor reproducibility among lots becomes problematic in production. In particular, a problem concerning the stability of the polymer having acetal as a protective group formed by reaction with isopropenyl methyl ether in the isolation and purification steps is reported by Uday Kumar, SPIE, Vol. 3049/135 (1997).
Although highly practical resist materials suitable for exposure with KrF excimer laser radiation have been found as described above, the great problems as described above remain in the conventional method for preparing the resist materials. Therefore, there is great demand for a method for preparing a resist material which is free of these problems, that is, which is suitable for large-scale production, has simplified steps, is effected economically and in a short time, does not pollute the environment, and gives a practical resist material excellent in resist characteristics; and a method for preparing a resist composition using the resist material obtained by this method.
As a result of their eager study and examination, the present inventors found that the degree of protection of the polymer can be controlled within a narrow range and the resist material can be prepared without isolation and purification of the polymer by using specific materials as a reaction catalyst, solvent and reaction stopper, for example by using none of aromatic compound as the acid catalyst in preparing the resist material etc. and by controlling the moisture content in the reaction system at low levels. By this, the problems in the prior art, that is, the problems of the environment, costs, complicated production process, variations in qualities among lots, and use of the unstable polymer as the resist material can be solved all at once, and the present invention is completed.
That is, an object of the present invention is to provide a practical method for preparing a resist composition, particularly a chemically amplified resist composition, which is suitable for exposure not only with deep-ultraviolet rays, KrF or ArF excimer laser radiation but also with electron beams, X-rays, i-line and g-line radiation, as well as a resist composition prepared by this method.
Another object of the present invention is to provide a method for preparing a resist composition wherein the resist composition can be obtained without isolating and purifying resist materials, as well as a resist composition prepared by this method.
An additional object of the present invention is to provide a method for preparing a resist composition economically and in a short time without using a large amount of solvent and water, as well as a resist composition produced by this method.
A further object of the present invention is to provide a method for preparing a resist composition hardly polluting the environment.